Dietary polyunsaturated fatty acids (PUFAs) have effects on diverse physiological processes impacting normal health and chronic diseases, such as the regulation of plasma lipid levels, cardiovascular and immune functions, insulin action, and neuronal development and visual function. Ingestion of PUFAs (generally in ester form, e.g. in glycerides or phospholipids) will lead to their distribution to virtually every cell in the body with effects on membrane composition and function, eicosanoid synthesis, cellular signaling and regulation of gene expression.
Variations in distribution of different fatty acids/lipids to different tissues in addition to cell specific lipid metabolism, as well as the expression of fatty acid-regulated transcription factors, is likely to play an important role in determining how cells respond to changes in PUFA composition. (Benatti, P. Et al, J. Am. Coll. Nutr. 2004, 23, 281).
PUFAs or their metabolites have been shown to modulate gene transcription by interacting with several nuclear receptors. These are the peroxisome proliferators-activated receptors (PPARs), the hepatic nuclear receptor (HNF-4), liver X receptor (LXR), and the 9-cis retinoic acid receptor (retinoic X receptor, RXR). Treatment with PUFAs can also regulate the abundance of many transcriptional factors in the nucleus, including SREBP, NFkB, c/EBPβ, and HIF-1α. These effects are not due to direct binding of the fatty acid to the transcription factor, but involve mechanisms that affect the nuclear content of the transcription factors.
The regulation of gene transcription by PUFAs have profound effects on cell and tissue metabolism and offer a credible explanation for the involvement of nutrient-gene interactions in the initiation and prevention or amelioration of diseases such as obesity, diabetes, cardiovascular disorders, immune-inflammatory diseases and cancers (Wahle, J., et al, Proceedings of the Nutrition Society, 2003, 349).
Fish oils rich in the omega-3 polyunsaturated fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), have been shown to reduce the risk of cardiovascular diseases partly by reduction of blood triglyceride concentration. This favorable effect mainly results from the combined effects of inhibition of lipogenesis by decrease of SPEBP-1 and stimulation of fatty acid oxidation by activation of PPAR-αin the liver.
Omega-3 polyunsaturated fatty acids in fish oil have been reported to improve the prognosis of several chronic inflammatory diseases characterized by leukocyte accumulation and leukocyte-mediated tissue injury, including atherosclerosis, IgA nephropathy, inflammatory bowel disease, rheumatoid arthritis, psoriasis, etc. (Mishra, A., Arterioscler. Thromb. Vasc. Biol., 2004, 1621).
Due to their limited stability in vivo and their lack of biological specificity, PUFAs have not achieved widespread use as therapeutic agents. Chemical modifications of the omega-3 polyunsaturated fatty acids have been performed by several research groups in order to change or increase their metabolic effects.
For example, the hypolipidemic effects of EPA was potentiated by introducing methyl or ethyl in α-position of EPA ethyl ester. (Vaagenes et. al Biochemical Pharm. 1999, 58, 1133).
In a recent work published by L. Larsen (Larsen, L. et al, Lipids, 2005, 40, 49) the authors show that the α-methyl derivatives of EPA and DHA increased the activation of the nuclear receptor PPAR and thereby the expression of L-FABP compared to EPA/DHA. The authors suggest that delayed catabolism of these α-methyl PUFAs contribute to their increased effects.
Nuclear receptors (NRs) constitute a large and high conserved family of ligand activated transcriptional factors that regulate diverse biological processes such as development, metabolism, and reproduction. It is recognized that ligands for these receptors might be used in the treatment of common diseases such as atherosclerosis, diabetes, obesity, and inflammatory diseases. As such, NRs have become important drug targets, and the identification of novel NR ligands is a subject of much interest.
The activity of many nuclear receptors is controlled by the binding of small, lipophilic ligands that include hormones, metabolites such as fatty acids, bile acids, oxysteroles and xeno- and endobiotics. Nuclear receptors can bind as monomers, homodimers, or RXR heterodimers to DNA.
The transcription factor NF-κB is an inducible eukaryotic transcription factor of the rel family. It is a major component of the stress cascade that regulate the activation of early response genes involved in the expression of inflammatory cytokines, adhesion molecules, heat-shock proteins, cyclooxygenases, lipoxygenases, and redox enzymes.
Zhao, G. et al (Biochemical and Biophysical Research Comm., 2005, 909) suggest that the anti-inflammatory effects of PUFAs in human monocytic THP-1 cells are in part mediated by inhibition of NF-κB activation via PPAR-γ activation. Others have suggested that the anti-inflammatory effect of PUFAs is mediated through a PPAR-α dependent inhibition of NF-κB activation.
To sum up, there is vast ongoing research in the field of PUFAs, and in particular omega-3 polyunsaturated fatty acids. However, their pharmaceutical potential has not yet been fully evaluated, and there is thus a continuing need for further evaluation and development of such compounds in order to increase their medical usefulness.